setproctitle_linux.c

// Copyright (c) 2009 The Chromium Authors. All rights reserved.// Use of this source code is governed by a BSD-style license that can be// found in the LICENSE file.// This file implements BSD-style setproctitle() for Linux.// It is written such that it can easily be compiled outside Chromium.//// The Linux kernel sets up two locations in memory to pass arguments and// environment variables to processes. First, there are two char* arrays stored// one after another: argv and environ. A pointer to argv is passed to main(),// while glibc sets the global variable |environ| to point at the latter. Both// of these arrays are terminated by a NULL pointer; the environment array is// also followed by some empty space to allow additional variables to be added.//// These arrays contain pointers to a second location in memory, where the// strings themselves are stored one after another: first all the arguments,// then the environment variables. The kernel will allocate a single page of// memory for this purpose, so the end of the page containing argv[0] is the// end of the storage potentially available to store the process title.//// When the kernel reads the command line arguments for a process, it looks at// the range of memory within this page that it initially used for the argument// list. If the terminating '\0' character is still where it expects, nothing// further is done. If it has been overwritten, the kernel will scan up to the// size of a page looking for another. (Note, however, that in general not that// much space is actually mapped, since argv[0] is rarely page-aligned and only// one page is mapped.)//// Thus to change the process title, we must move any environment variables out// of the way to make room for a potentially longer title, and then overwrite// the memory pointed to by argv[0] with a single replacement string, making// sure its size does not exceed the available space.//// It is perhaps worth noting that patches to add a system call to Linux for// this, like in BSD, have never made it in: this is the "official" way to do// this on Linux. Presumably it is not in glibc due to some disagreement over// this position within the glibc project, leaving applications caught in the// middle. (Also, only a very few applications need or want this anyway.)#include "base/setproctitle_linux.h"#include <stdarg.h>#include <stdint.h>#include <stdio.h>#include <string.h>#include <unistd.h>externchar** environ;
staticchar** g_main_argv = NULL;
staticchar* g_orig_argv0 = NULL;
void setproctitle(constchar* fmt, ...) {
va_list ap;
size_t i, avail_size;
uintptr_t page_size, page, page_end;
// Sanity check before we try and set the process title.// The BSD version allows fmt == NULL to restore the original title.if (!g_main_argv || !environ || !fmt)
return;
if (!g_orig_argv0) {
// Save the original argv[0].
g_orig_argv0 = strdup(g_main_argv[0]);
if (!g_orig_argv0)
return;
}
page_size = sysconf(_SC_PAGESIZE);
// Get the page on which the argument list and environment live.
page = (uintptr_t) g_main_argv[0];
page -= page % page_size;
page_end = page + page_size;
// Move the environment out of the way. Note that we are moving the values,// not the environment array itself (which may not be on the page we need// to overwrite anyway).for (i = 0; environ[i]; ++i) {
uintptr_t env_i = (uintptr_t) environ[i];
// Only move the value if it's actually in the way. This avoids// leaking copies of the values if this function is called again.if (page <= env_i && env_i < page_end) {
char* copy = strdup(environ[i]);
// Be paranoid. Check for allocation failure and bail out.if (!copy)
return;
environ[i] = copy;
}
}
// Put the title in argv[0]. We have to zero out the space first since the// kernel doesn't actually look for a null terminator unless we make the// argument list longer than it started.
avail_size = page_end - (uintptr_t) g_main_argv[0];
memset(g_main_argv[0], 0, avail_size);
va_start(ap, fmt);
if (fmt[0] == '-') {
vsnprintf(g_main_argv[0], avail_size, &fmt[1], ap);
} else {
size_t size = snprintf(g_main_argv[0], avail_size, "%s ", g_orig_argv0);
if (size < avail_size)
vsnprintf(g_main_argv[0] + size, avail_size - size, fmt, ap);
}
va_end(ap);
g_main_argv[1] = NULL;
}
// A version of this built into glibc would not need this function, since// it could stash the argv pointer in __libc_start_main(). But we need it.void setproctitle_init(char** main_argv) {
if (g_main_argv)
return;
uintptr_t page_size = sysconf(_SC_PAGESIZE);
// Check that the argv array is in fact on the same page of memory// as the environment array just as an added measure of protection.if (((uintptr_t) environ) / page_size == ((uintptr_t) main_argv) / page_size)
g_main_argv = main_argv;
}